The invention relates to a measuring device that can be connected to an internal combustion engine and function as a tachometer to determine engine speed. More specifically, the invention is an interface circuit that is positioned between the measuring device and the internal combustion engine such that the ignition pulses from the internal combustion engine are conditioned and can be measured by the measuring device.
In the field of internal combustion engines, a hand-held, multi-functional measuring device is often used to determine various operating parameters throughout the engine. The hand-held measuring device includes a pair of lead wires that can be attached to various points in the engine at which the operating characteristics of the engine need to be determined. Typically, the hand-held measuring device includes a selection dial that allows a technician to select which kind of measurement is to be made by the measuring device. For example, the measuring device could include settings to measure resistance, voltage, current and other operating conditions, such as the engine speed. When the measuring device is acting as tachometer to measure engine speed, the leads from the measuring device are applied to the ignition system of the internal combustion engine, and the device measures the frequency of the ignition pulses.
The tachometer function of commonly available multi-functional measuring devices is configured to be used on an automotive internal combustion engine. Typically, the ignition system in an automotive engine is an inductive ignition system. Thus, the measuring device is internally configured to measure signals from an inductive ignition system. When the measuring device is properly connected to the engine, the measuring device receives the series of ignition pulses from the internal combustion engine and displays the engine speed on a display panel.
While this type of widely available measuring device is constructed to be used with automotive internal combustion engines, it is also desirable to use the multi-functional measuring device in the marine industry, since similar engine characteristics are important when working on a marine engine. However, marine engines are constructed somewhat differently than their automotive counterparts, and thus behave differently. For example, some marine engines include capacitive discharge ignition systems that produce ignition pulses that are different from those produced by an inductive ignition system found in the automotive industry. When the multi-functional device is connected to an inductive ignition system, the multifunctional measuring device is unable to make an accurate and dependable engine speed measurement. Additionally, the ignition pulses in a marine engine include a larger noise component that makes measurement of the ignition pulses more difficult when compared to an automotive engine.
Since multi-functional measuring devices are currently available and are mass produced, the price of the measuring device is relatively inexpensive. However, since the measuring device oftentimes cannot accurately determine engine speed for many marine engines, it has been found to be unusable for many applications.
Therefore, a tachometer interface circuit that can be used to condition the ignition pulses from a variety of marine internal combustion engines such that the ignition pulses can be read by the standard multi-functional measuring device would be particularly desirable in the marine propulsion industry. Specifically, a tachometer interface circuit that can be inserted between the measuring device and the leads which are attached to the internal combustion engine would be particularly desirable.